Numerous catalyst systems for use in the polymerization of ethylenically unsaturated monomers are based upon metallocenes. Metallocenes can be characterized generally as coordination compounds incorporating one or more cyclopentadienyl groups (which may be substituted or unsubstituted) coordinated with a transition metal. Various types of metallocenes are known in the art. They include bicyclic coordination compounds of the general formula:(Cp)2MeQn  (1)characterized by the isospecific metallocenes as described below and dicyclopentadienyl compounds of the general formula:Cp Cp′MeQn  (2)characterized by the syndiospecific metallocenes described below. In the aforementioned formulas the Me denotes a transition metal and Cp and Cp′ each denote a cyclopentadienyl group which can be either substituted or unsubstituted with Cp′ being different from Cp, Q is an alkyl or other hydrocarbyl group or a halogen and n is a number within the range of 1–3. The cyclopentadienyl groups are in a stereorigid relationship normally provided by a bridged structure between the metallocene rings (not shown in Formulas (1) and (2) above) although stereorigidity can be provided through substituent groups which result in steric hindrance, as described, for example, in U.S. Pat. No. 5,243,002 to Razavi. Also, while bridged metallocenes normally incorporate two cyclopentadienyl groups (or substituted cyclopentadienyl groups), bridged metallocenes incorporating a single cyclopentadienyl group which is bridged to a heteroatom aromatic group (both being coordinated with a transition metal) are also known in the art. For example, U.S. Pat. No. 5,026,798 to Canich discloses dimethylsilyl-bridged cyclopentadienyl—anilino or other heteroatom ligand structures with coordination to the transition metal being provided through the nitrogen atom of the anilino group.
As noted previously, isospecific and syndiospecific metallocene catalysts are useful in the polymerization of stereospecific propagation of monomers. Stereospecific structural relationships of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers from various monomers. Stereospecific propagation may be applied in the polymerization of ethylenically unsaturated monomers such as C3+ alpha olefins, 1-dienes such as 1,3-butadiene, substituted vinyl compounds such as vinyl aromatics, e.g, styrene or vinyl chloride, vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g., isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific polymer propagation is probably of most significance in the production of polypropylene of isotactic or syndiotactic structure.
The structure of isotactic polypropylene can be described as one having the methyl groups attached to the tertiary carbon atoms of successive monomeric units falling on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene is described as follows:
In structure (3) each vertical segment indicates a methyl group on the same side of the polymer backbone. Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad as shown above is . . . mmmm . . . with each “m” representing a “meso” dyad, or successive pairs of methyl groups on the same said of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene in using the Fisher projection formula can be indicated by racemic dyads with the syndiotactic pentad rrrr shown as follows:
Here, the vertical segments again indicate methyl groups in the case of syndiotactic polypropylene, or other terminal groups, e.g. chloride, in the case of syndiotactic polyvinyl chloride, or phenyl groups in the case of syndiotactic polystyrene.
Syndiotactic polymers are semi-crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is non-crystalline and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
Yet another polymer configuration which has both isotactic and atactic features is exemplified by hemi-isotactic polypropylene. Hemi-isotactic polypropylene is characterized by every other methyl group being on the same side of the polymer with the remaining methyl groups randomly being on the same side or on the opposite side of the polymer backbone. Hemi-isotactic polypropylene can be characterized by the following Fisher projection formula in which, as indicated by the broken lines, alternate methyl groups have random steric configurations.
Thus, as shown in structure (5), the methyl groups indicated by the solid lines are in a mesa relationship with one another, with the alternating methyl groups indicated by the broken lines being randomly configured. Hemi-isotactic polypropylene, while having a semi-ordered structure, is primarily non-crystalline because of the disorder of the alternate methene units.
In most cases, the preferred polymer configuration will be a predominantly isotactic or syndiotactic polymer with very little atactic polymer. Catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes which may be characterized by the following formula:R″(C5(R′)4)2 HfQp  (6)In formula (7), (C5(R′)4) is a cyclopentadienyl or substituted cyclopentadienyl group, R′ is independently hydrogen or a hydrocarbyl radical having 1–20 carbon atoms, and R″ is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1–20 carbon atoms and p is 2.
Catalysts that produce syndiotactic polypropylene or other syndiotactic polyolefins and methods for the preparation of such catalysts are disclosed in the aforementioned U.S. Pat. No. 4,892,851. These catalysts are also bridged stereorigid metallocene catalysts, but in this case, the catalysts have a structural bridge extending between dissimilar cyclopentadienyl groups and may be characterized by the formula:R″(CpRn) (CpR′m)MeQk  (7)In formula (7), Cp represents a cyclopentadienyl or substituted cyclopentadienyl ring, and R and R′ represent hydrocarbyl radicals having 1–20 carbon atoms. R″ is a structural bridge between the rings imparting stereorigidity to the catalyst. Me represents a transition metal, and Q a hydrocarbyl radical or halogen. R′m is selected so that (CpR′m) is a sterically different substituted cyclopentadienyl ring that (CpRn). In formula (8) n varies from 0–4 (0 designating no hydrocarbyl groups, i.e., an unsubstituted cyclopentadienyl ring), m varies from 1–4, and k is from 0–3. The sterically different cyclopentadienyl rings produce a predominantly syndiotactic polymer rather than an isotactic polymer. Like their isospecific counterparts, the syndiospecific metallocene disclosed in the '851 patent are used in combination with aluminum co-catalysts. The aluminum co-catalyst may take the form of an alumoxane and trialkyl aluminum or mixtures thereof. Specifically disclosed in the '851 patent is methylalumoxane and triethylaluminum (TEAL).
Specifically disclosed in U.S. Pat. No. 4,892,851 are bridged metallocene ligands having a dissimilar cyclopentadienyl group resulting from the reaction of 6,6-dimethyl fulvene with a substituted cyclopentadiene fluorene to produce a ligand characterized by an isopropylidene bridge structure. Preferably, this ligand structure is characterized as having bilateral symmetry such as indicated by the isopropylidene(cyclopentadienyl fluorenyl) structure as shown below:
As indicated by formula (8), the bilateral symmetry of the ligand structure is indicated by the balanced orientation about the broken line representing a plane of symmetry extending generally through the bridge structure and the transition metal atom.
The foregoing structure may be contrasted with a metallocene which lacks bilateral symmetry which can be used in the production of hemi-isotactic polypropylene as described in the U.S. Pat. No. 5,036,034 to Ewen. An example of a compound indicating a lack of bilateral symmetry is isopropylidene (3-methyl cyclopentadienyl-1 fluorenyl) zirconium dichloride having the ligand structure shown by the following formula:
As explained in more detail in the aforementioned Ewen '034 patent, the lack of bilateral symmetry is indicated by the right side of the structure relative to the broken line being different from the left side because of the methyl group substituted at the distal position on the cyclopentadienyl group.
The various metallocene structures as described above can be used either as so-called “neutral metallocenes” in which case an alumoxane, such as methylalumoxane, is used as a co-catalyst, or they can be employed as so-called “cationic metallocenes” which incorporate a stable non-coordinating anion and normally do not require the use of an alumoxane. Syndiospecific cationic metallocenes are disclosed for example in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, the metallocene cation is characterized by the cationic metallocene ligand having sterically dissimilar ring structures which are joined to a positively-charged coordinating transition metal atom. The metallocene cation is associated with a stable non-coordinating counter-anion. Co-catalysts, such as disclosed in the aforementioned Ewen '851 and Ewen '034 patents, may also be employed with cationic metallocenes such as disclosed in the Razavi '002 patent. As disclosed in Razavi, alumoxanes such as methylalumoxanes normally will not be employed. However, alkyl aluminums, such as TEAL described previously, and other co-catalysts, such as trimethylaluminum, trialkoxy aluminums, dialkylaluminum halides, or alkylaluminum dihalides, may be employed as disclosed in the Razavi '002 patent.
The aforementioned Razavi '002 patent also discloses the establishment of a stereorigid relationship imparted by a sterically hindered relationship between substituted cyclopentadienyl rings which prevent rotation of the ring structures about their coordination axis. Alternatively, the cyclopentadienyl groups may be highly substituted such that a relatively low kinetic energy state is induced by the substituents in order to prevent rotation rings about their coordination axis at the temperature of the catalyst. Such cationic metallocenes also may, of course, like their neutral counterparts, be characterized by a stereorigid relationship imparted by means of a structural bridge between the cyclopentadienyl groups.
U.S. Pat. No. 5,225,500 to Elder et al. discloses stereorigid cationic metallocenes, including, inter alia, bridged metallocene catalysts useful for the production of syndiotactic polymers. The bridged metallocene catalysts of U.S. Pat. No. 5,225,500 comprise an unbalanced metallocene cation and a stable, non-coordinating counteranion for the metallocene cation. The metallocene cation is characterized by a cationic metallocene ligand having sterically dissimilar ring structures joined to a positively charged coordinating transition metal atom. The dissimilar cyclopentadienyl rings, at least one of which is substituted, are both in a stereorigid relationship relative to the coordinating metallocene of the metal atom catalyst, and, as noted previously, the stereorigid relationship may be imparted by means of a structural bridge between the dissimilar cyclopentadienyl rings.
While metallocene catalysts are often used as homogeneous catalysts, it is also known in the art to provide supported metallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst component may be employed in the form of a supported catalyst. As described in the Welborn '432 patent, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesia, titania, zirconia and the like. Non-metallocene transition metal compounds, such as titanium tetrachloride, are also incorporated into the supported catalyst component. The inorganic oxides used as support are characterized as having an average particle size ranging from 30–600 microns, preferably 30–100 microns, a surface area of 50–1,000 m2/g, preferably 100–400 m2/g, a pore volume of 0.5–3.5 cm3/g, preferably about 0.5–2 cm3/g. Generally, the particle size, surface area, pore volume, and number of surface hydroxyl groups are said to be not critical to the Welborn procedure. Specifically disclosed in Welborn is a catalyst in which bis(cyclopentadienyl)zirconium dichloride (unbridged metallocene) is supported on a high surface area silica dehydrated in dry nitrogen at 600° C. and characterized as Davison D-952. The Welborn '561 patent discloses a heterogeneous catalyst which is formed by the reaction of a metallocene and an alumoxane in combination with the support material. The support in Welborn '561 is described similarly as the support in the Welborn '432 patent. A catalyst system embodying both a homogeneous metallocene component and a heterogeneous component, which may be a “conventional” supported Ziegler-Natta catalyst, e.g. a supported titanium tetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Shamshoum et al. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. Nos. 5,308,811 to Suga et al. and 5,444,134 to Matsumoto. In both patents, the supports are characterized as various high surface area inorganic oxides or clay-like materials. In the patent to Suga et al., the support materials are characterized as clay minerals, ion-exchanged layered compounds, diatomaceous earth, silicates or zeolites. As explained in Suga, the high surface area support materials should have volumes of pores having radii of at least 20 angstroms. Specifically disclosed and preferred in Suga are clay and clay minerals such as montmorillonite. The catalyst components in Suga are prepared by mixing the support material, the metallocene, and an organoaluminum compound such as triethylaluminum, trirethylaluminum, various alkylaluminum chlorides, alkoxides, or hydrides or an alumoxane such as methylalumoxane, ethylalumoxane, or the like. The three components may be mixed together in any order, or they may be simultaneously contacted. The patent to Matsumoto similarly discloses a supported catalyst in which the support may be provided by inorganic oxide carriers such as SiO2, Al2O3, MgO, ZrO2, TiO2, Fe2O3, B2O2, CaO, ZnO, BaO, ThO2 and mixtures thereof, such as silica alumina, zeolite, ferrite, and glass fibers. Other carriers include MgCl2, Mg(0-Et)2, and polymers such as polystyrene, polyethylene, polypropylene, substituted polystyrene and polyarylate, starches, and carbon. The carrier has a surface area of 1–1,000 m2/g, preferably 50–500 m2/g, a pore volume of 0.1–5 cm3/g, preferably 0.3–3 cm3/g, and a particle size of 20–100 microns.
Of the various inorganic oxides used as supports, silica, in one form or another, is widely disclosed as a support material for metallocene catalysts. Silica, characterized as Davison D-948 or Davison D-952, appears as a conventional metallocene support. For example, U.S. Pat. No. 5,466,649 to Jejelowa discloses the use of dehydrated Davison D-948 silica as a support for various unbridged metallocenes used in conjunction with supported co-catalysts. U.S. Pat. No. 5,498,581 to Welch et al. discloses silica for use as a support for either bridged or unbridged metallocenes in which the silica is treated with carbon monoxide, water, and hydroxyl groups to inactive species. Specifically disclosed is the silica, Davison D-948, having an average particle size of 50 microns. Other silica-based supports are disclosed in U.S. Pat. Nos. 5,281,679 to Jejelowa, 5,238,892 to Chang, and 5,399,636 to Alt. The Chang and Jejelowa patent disclose the use of a silica support identified as Davison D-948, which is characterized as an amorphous silica gel containing about 9.7 wt. % water. As described in the Chang and Jejelowa patents, alumoxane is formed directly on the surface of the silica gel by direct reaction of an alkyl aluminum with silica gel which is undehydrated so as to ensure the conversion of the quantity of the alkyl aluminum to an alumoxane that has a high degree of oligomerization. The water-impregnated gel is characterized as having a surface range of 10–700 m2/g, a pore volume of about 0.5–3 cm3/g, and an absorbed water content of from about 10–50 wt. % in the case of the Jejelowa patent, and about 6–20 wt. % in the case of the Chang patent. The average particle size for the silica is described in Chang to be from 0.3–100 microns and in Jejelowa from about 10–100 microns. After the alumoxane silica gel component has been formed, the metallocene may be added to the wet slurry.
Other supported catalyst systems are disclosed in U.S. Pat. No. 6,166,153 to Shamshoum et al. Here, a silica support such as described above is pretreated with an alumoxane, such as methylalumoxane, followed by addition of a syndiospecific metallocene on the MAO-treated silica. The supported metallocene is used in conjunction with an organo-aluminum co-catalyst such as a monoalkyl or dialkyl aluminum halides as described previously, including trialkylaluminums such as trimethylaluminum, triethylaluminum or tri-isobutyl aluminum (TIBAL). In the supported catalyst disclosed in EP 819706, the silica support is a high surface area, small pore size silica which is first dried, slurried in a non-polar solvent, and then contacted with methylalumoxane in a solvent. The metallocene was then dissolved in a non-polar solvent, particularly the same as used as the solvent for the alumoxane. The solid metallocene supported on the alumoxane-treated silica is then recovered from the solvent, dried, and then incorporated into carrier liquid such as mineral oil. The Shamshoum European application also discloses a pre-polymerization step which can be used to decrease the aging time of the catalyst in the trialkyl aluminum or other aluminum co-catalyst.